As one of methods of improving the frictional wear property of a resinous article, so-called “gan-pla” (lubricating oil-containing plastic) of adding lubricating oil thereto is known. In this method, the resin and the lubricating oil are melted, kneaded, pelletized to obtain a mixture thereof having a certain configuration, and injection-molded or extrusion-molded. In this method, to prevent deterioration of the injection moldability, the amount of the lubricating oil which can be added to the resin is up to about 10 vol %. Because the resin and the lubricating oil are melted and kneaded, the resinous article has a high uniformity and oil-retaining performance, and the lubricating oil little bleeds out to the surface thereof. Therefore the function of the resinous article in supplying the lubricating oil is insufficient. Further when the resinous article is used as a sliding bearing, a sufficient frictional performance is not obtained in a region in which a PV value which is the product of a load (P) and a sliding speed (V) is high.
Heretofore as an oil-containing resin, developed to solve the above-described problem, which is a mixture of resin and lubricating oil to be molded into a predetermined configuration, the composition containing the ultra-high-molecular-weight polyethylene and the grease is disclosed (see patent document 1). Because the grease of the composition prevents the lubricating oil from flowing, the lubricating oil can be contained in the resin at not less than 50 vol %. The amount of bleeding of the lubricating oil to the surface of the composition is larger than the amount of bleeding of the lubricating oil to the surface of the resinous article formed by the gan-pla.
There is disclosed the oil-containing resin to which the oil guide material is added (see patent documents 2 through 4); the lubricating oil retained by the porous silica is mixed with the synthetic resin to obtain the oil-containing resin (see patent document 5). These oil-containing resins are intended to successively supply the surface of the resin with the lubricating oil. Owing to the help of the guide article and the porous silica, the amount of the lubricating oil that can be contained in the resin is increased to about 20 vol %.
A method of impregnating the resin molded by sintering it into a predetermined configuration with the lubricating oil is disclosed (see patent documents 6 and 7). In consideration of closest packing and compression in a molding step, a porosity which is a substantial ratio of a void is up to 30%. The impregnating oil can be contained in the void.
In the patent document 1, although a sufficient amount of the oil is secured, the composition is of an incorporating type of mixing the resin and the lubricating agent with each other. Therefore the composition has a high oil-retaining performance and a low utilization efficiency of the added lubricating oil. The incorporating type has a problem that when the amount of the lubricating oil is large, the mechanical strength of the oil-containing resinous article becomes low.
In the oil-containing resins disclosed in the patent documents 2, 3, 4, and 5, as described above, owing to the oil guide materials and the porous silica, the amount of the lubricating oil in the resin is increased up to 20 vol %. Further owing to the addition of the reinforcing agent, a decrease of the mechanical strength of the oil-containing resins can be restrained. But when the oil-containing resins are applied to a retainer of a bearing to lubricate it with only the oil in the resin, there occur problems that the amount of the lubricating oil is insufficient and that the bleeding speed thereof to the surface is low.
Further the methods disclosed in the patent documents 1 through 5 are intended to shape the kneaded resin and lubricating oil into predetermined configurations. Therefore the methods have a limitation in the combination of the resin and the lubricating oil and are unapplicable to a wide range of use.
On the other hand, in the methods disclosed in patent documents 6 and 7, the degree of freedom in the combination of the resin and the lubricating oil is increased. But the resin which can be actually molded by sintering it is limited to ultra-high-molecular-weight polyethylene and polyimide resin. As described above, the substantial porosity is up to 30%. Thus when the oil-containing resins are applied to the retainer of the bearing to lubricate it with only the oil in the resin, the amount of the lubricating oil is insufficient. Thus the oil-containing resins are incapable of satisfying a recent-demand for a long life.
A desalting method is known as a method of allowing adjustment of the porosity and production of an inexpensive resinous porous article. The desalting method is the method of producing the resinous porous article by molding a material composed of resin or rubber to which a powdery pore-forming substance such as sodium chloride and sodium sulfate is added as a solid molding containing the pore-forming substance, eluting the pore-forming substance by cleaning the obtained solid molding with water, and forming pores at a portion where the pore-forming substance has been present.
The following methods of producing the resinous porous article having a high porosity by using the desalting method are known: The method, of producing the resinous porous article, which is carried out by using the pore-forming substance which is solid at a normal temperature and melts at the molding temperature of the polymeric substance forming the skeleton of the resinous porous article and is thus capable of being present in a liquid state (see patent document 8); the method of forming pores by molding the material composed of a particulate pore-forming substance dispersed in the polymeric substance at the temperature at which a part of the particulate pore-forming substance dissolves to form the porous article and washing the porous article with the solvent not dissolving the polymeric substance therein but dissolves the pore-forming substance therein (see patent document 9); and the method of producing the porous article made of polyolefin resin having interconnected bubbles (see patent document 10).
To facilitate the separation of an extraction and reuse of a material, the pore-forming substance consisting of water-soluble powder is extracted by using hot water (see patent document 11).
Sodium chloride, ammonium chloride, sodium sulfate, sodium nitrate, potassium sulfate, magnesium sulfate, and calcium chloride disclosed as the pore-forming substance in the above-described patent documents are comparatively water-soluble, inexpensive, and easily available. Thus they are effective as the pore-forming substance to be used in the production of the resinous porous article having a large pore diameter. But in forming fine pores, it is difficult to completely dissolve and extract the pore-forming substance.
Therefore, an unextracted portion of the pore-forming substance bleeds during operation and may rust the iron and steel, when the resinous porous article is used as parts which contact iron and steel. Therefore when the resinous porous article is impregnated with the lubricating oil to use it as a resinous oil-retaining article, its use as a rolling bearing, a sliding bearing, and the like around which the iron and steel is present is inappropriate.
In the desalting method, the size of the powdery pore-forming substance determines the size of the pore unlike a pore-forming method using a foaming agent. Thus it is necessary to control the size of the powdery pore-forming substance. It is also necessary to control the diameter of particles of the pore-forming substance according to the diameter of the particles thereof or the kind of the article thereof by classifying the particles thereof. Thus the resinous porous article produced by the desalting method is expensive.
Patent document 1: Japanese Patent Application Laid-Open No. 6-41569
Patent document 2: Japanese Patent Application Laid-Open No. 11-166541
Patent document 3: Japanese Patent Application Laid-Open No. 2000-71243
Patent document 4: Japanese Patent Application Laid-Open No. 2000-71244
Patent document 5: Japanese Patent Application Laid-Open No. 2002-129183
Patent document 6: Japanese Patent Application Laid-Open No. 61-6429
Patent document 7: Japanese Patent Application Laid-Open No. 9-76371
Patent document 8: Japanese Patent Application Laid-Open No. 2001-2825
Patent document 9: Japanese Patent Application Laid-Open No. 2002-194131
Patent document 10: Japanese Patent Application Laid-Open No. 2002-60534
Patent document 11: Japanese Patent Application Laid-Open No. 2002-322310